Assessment of ultrashort echo time (UTE) T* mapping at 3T for the whole knee: repeatability, the effects of fat suppression, and knee position.

Background: Knee tissues such as tendon, ligament and meniscus have short T* relaxation times and tend to show little to no signal in conventional magnetic resonance acquisitions. An ultrashort echo time (UTE) technique offers a unique tool to probe fast-decaying signals in these tissues. Clinically relevant factors should be evaluated to quantify the sensitivity needed to distinguish diseased from control tissues. Therefore, the objectives of this study were to (I) quantify the repeatability of UTE-T* relaxation time values, and (II) evaluate the effects of fat suppression and (III) knee positioning on UTE-T* relaxation time quantification.

Methods: A dual-echo, three-dimensional center-out radially sampling UTE and conventional gradient echo sequences were utilized to image gadolinium phantoms, one specimen, and five subjects on a clinical 3T scanner. Scan-rescan images from the phantom and experiments were used to evaluate the repeatability of T* relaxation time values. Fat suppressed and non-suppressed images were acquired for phantoms and the specimen to evaluate the effect of fat suppression on T* relaxation time quantifications. The effect of knee positioning was evaluated by imaging subjects in extended and flexed positions within the knee coil and comparing T* relaxation times quantified from tissues in each position.

Results: Phantom and measurements demonstrated repeatable T* mapping, where the percent difference between T* relaxation time quantified from scan-rescan images was less than 8% for the phantom and knee tissues. The coefficient of variation across fat suppressed and non-suppressed images was less than 5% for the phantoms and knee tissues, showing that fat suppression had a minimal effect on T* relaxation time quantification. Knee position introduced variability to T* quantification of the anterior cruciate ligament, posterior cruciate ligament, and patellar tendon, with percent differences exceeding 20%, but the meniscus showed a percent difference less than 10%.

Conclusions: The 3D radial UTE sequence presented in this study could potentially be used to detect clinically relevant changes in mean T* relaxation time, however, reproducibility of these values is impacted by knee position consistency between scans.